Spectroscopy is the study of the interaction between electromagnetic radiation and a sample (e.g., containing one or more of a gas, solid and liquid). The manner in which the radiation interacts with a particular sample depends upon the properties (e.g., molecular composition) of the sample. Generally, as the radiation passes through the sample, specific wavelengths of the radiation are absorbed by chemical compounds within the sample. The specific wavelengths of radiation that are absorbed are unique to each of the chemical compounds within the specific sample. By identifying which wavelengths of radiation are absorbed, it is therefore possible to identify the specific chemical compounds present in the sample.
Infrared spectroscopy is a particular field of spectroscopy in which, for example, the types of chemical compounds and the concentration of individual chemical compounds within a sample are determined by subjecting the sample (e.g., gas, solid, liquid or combination thereof) to infrared electromagnetic energy. Generally, infrared energy is characterized as electromagnetic energy having wavelengths of energy between about 0.7 μm (frequency 14,000 cm−1) and about 1000 μm (frequency 10 cm−1). Infrared energy is directed through the sample and the energy interacts with the chemical compounds within the sample. The energy that passes through the sample is detected by a detector (e.g., an electromagnetic detector). The detected signal is then used to determine, for example, the molecular composition of the sample and the concentration of specific chemical compounds within the sample.
For infrared spectroscopy, the infrared absorbance spectrum can be linked to chemical concentrations by mathematical equations (e.g., Beer's Law, which relates the absorption of light to the properties of the material through which the light is traveling). A critical variable in these equations is the “background spectrum” (or “background spectra”), which can be used as a baseline from which to detect and quantify new chemical compounds. The background spectrum is often calculated to quantify the infrared source that is being passed through the sample of interest. The background spectrum can also account for other instrument functions and environmental conditions. For example, the background spectrum can be measured in a clean environment before a sample of interest is introduced to the system, such that new measurements are compared against the background spectrum. Advantageously, the system can ignore the background chemical compounds and other background components (identified using the background spectrum) and only monitor and/or detect and quantify new (or additional) chemical compounds.
The background spectrum is often approximated by a single background spectrum (e.g., a constant background spectrum that does not compensate for changes over time, such as changes to the instrument and/or the environment). Ideally, when the instrument's stability and environmental conditions are well controlled, the actual background spectra should be a constant vector over time that is sufficiently close enough to the background spectrum such that the error is white and negligible (and the chemical identification and quantification of new chemical compounds can be implemented with acceptable accuracy and precision). However, the instrument and/or the environment often change so frequently that a single background spectrum does not accurately model the true background of the system. This can lead to inaccurate and/or erroneous chemical compound detections (e.g., false positives or missed chemical compound detections).